Method of treating metallic articles



,hard surface on metal articles.

PatentedF eb. .13, 1940 UN lTE. smrss FATE METHOD OF TREATING. METALLICARTICLES I Lyndon St Tracy, Syracuse, N. Y., assignor to The Solvay'Process Company, New York, N. Y., a

corporation of New York No Drawing. Application April 22, 1938, I

Serial No. 203,587

, v 2 Claims. My invention relates to theeproduction of a The inventionis particularly directed to producing a hard,

abrasion and corrosion-resistant surface on iron and steel. 5

In many industrial operations it is necessary to protect metallicequipment against excessive By applying a hard surface or facing towear. protect the underlying metaL'the life and efliciencyofsuchmetallic equipment can be increased. .For example, pumps, pipe lines,and other apparatus for handling corrosive and erosive materials can bemade more durable by providing an abrasionand corrosion-resistantfacingon their exposed surfaces. v

It is' well known that various carbides are extremely hard and abrasivecompounds, and in' the past certain carbides, such as tungsten carbide,"chromium carbide, tantalum carbide and vanadium carbide,'have beensuggested for hardening steel by fusion of the carbide onto the surfaceof the steel. So far as is known, all of the carbides suggested by theprior art have been metallic carbides and, while certain of them, for,example tungsten carbide, have been generally satisfactory, theyarenevertheless rela tivelyqexpensive.

The; high cost attending the use of, metallic carbides is wellrecognized and attemptshave been made to reduce this cost.

For instance, it has been suggested to apply a relatively cheap ore ofthe metal and some form ofcarbon to {the surface of the steel and thenapply heat to form the metal carbide, whereupon it is fused into thesteel. An excess of carbon,

an atmosphere of hydrogen, orsomeother reducing agent must besupplied inthis latter processto first effect a" reduction of the ore to the puremetal. An analogous process has been suggested which eliminates thisinitial reduction step by starting withan uncombined form' of the metalin place of the ore. However, as the uncpmbined metal is a comparativelyexpensive,

(01.148-) I H carbide, a non-metallic carbide; intothe surface 1thereof. j

The product of my invention is asolid metal body, of Whatever desiredform, having a high concentration of silicon and carbide at. thesurface. 'It is a unitary mass in that the portion having the highsilicon and carbide content a part of rather than separate from themetal body, 4: Q a Although the use of silicon carbide as a packchangein carbon content of the steel has been suggested, the silicon carbideused in this way and the surface thus produced does not possess C thehigh corrosion and erosion resistance obtaindces not become a part ofthe treated surface able by the process o'f the present invention.

In the practice of my invention comminuted silicon carbide is applied to(the metal surface until it begins to fuse; The heating is continued NTf 1o ing material during heattempering of high speed steel (atrelatively low temperature), to prevent until the silicon carbideapparently goes into solution. After the metal ,coolsit is found to havea veryhardisurface resistant to abrasion and to corrosion by salineliquors.

The depth of the layer may be on "the order of k inch to 4; inch. Witha'much thinner layer it becomes difficult to provide a uniform '30coating, whereas a thicker layer requires longer heating. [Variations inthickness of the layer are also reflected in the depth of the hardprotective' alloy but the depths of hard alloy pro vided by thicknesseswithin the range specified have been found entirely satisfactory inpractice. The size of the silicon carbideparticles applied to the metalsurface may vary between wide limits; for instance,- they may. beparticles of 9 inch mesh down to the finest powder.

better during fusion; small particles, on the other hand, are dissolvedmore rapidly and may provide a slightly more uniform surface.

Articles treated by the process of my invention may be distinguished bythe fact that they have their silicon, and carbide content concentratedalong thetreated surface, as a layer, which. may be between 0.05 inchand 0.4 inch in thickness. Thus a steel article containing less, than0.4% ofsilicon and less than 0.4% of carbon,

when treated by the process of the present invention may have thesilicon content raised to between 1% and 4% and the carbon content tobetween 0.6% and 2% alongits surface without aifecting the silicon andcarbon content of Large particles are usually cheaper and stay in place20. to be hardened. Themetal surface is then heated the metal massbeyond the depth of the treated layer.

In the preferred embodiment of my invention, I employ an electric arefor efiecting the fusion of the metal, the are being struck between asuitable electrode, and the metal surface being treated. Nitrogenintroduced into the metal from the ionized atmosphere in the vicinity ofthe arc increases the hardness of the metal surface.

The invention will be further illustrated by the following specificexample, but it is understood the invention is not limited thereto, as

there may be wide deviations without departing from the spirit or scopeof the invention.

Number 24 grain silicon carbide"- was spread in a layer about one-eighthof an inch thick on the surface of a cast carbon-steel pump casingplate, the metal of which was about 'oneinch thick. The plate was atnormal room tempera ture. An electric arc of 200 to 250 amperes wasstruckfroma pointed inch diameter carbon electrode through'the layer ofcarborundum to the steel. The electrode was played back and forthover asmall area (about aninch) of the surface until the steel in this areawas fused and formed a small puddle of molten steel with the siliconcarbide floating on the surface. After a" few seconds the siliconcarbide seemed suddenly to be swallowed up by the liquid metal. The arewas then moved onto solid metal at the edgeo'f-the puddle. Thisprocedure was repeated until the whole surfaceof the steel plate hadbeen treated. As the arc was moved away from each puddle of liquid, thelatter cooled Within a few seconds sufiiciently to solidify since theheat was rapidly removed by normal radiation and especially byconduction to the rela-.

, alloy facing had an average depth of about A;

inch, i v

Comparative analyses of afacing madev in the .above manner upon a 1 inchplate of tank steel I Facing Plate Percent Percent Silicon. Y i 2. 7l 0.l3 Oarbon 0. 96 0.15 Nitrogen" 0.035 0. 004

Steel which" has been hardened by my invention' has been found to bewell adapted for the manufacture of centrifugal pumps designed forpumping corrosive and erosive chemicals such as the hot, saline wasteliquors from the ammonia still of a crude sodium bicarbonatemanufacturing plant. These waste liquors containsodium and calciumchlorides in solution and are somewhat corrosive to steel. Moreover,they contain solid calcium carbonate, lime, and sandy material derivedfrom the lime, and are so highly abrasive that they rapidly erodeordinary (carbon) steel pump casings.

A carbon-steel pump casing hardened with silicon carbide in accordancewith my invention Was still satisfactory after 105 operating daysDumping waste liquors from an ammonia still and could be expected tolast much longer. Cast iron pumps operating under similar conditions aresatisfactory for only about 30 operating days,

as an average.

The present invention may be used for providing a hard surface for metalobjects in general which are to be subjected to highly corrosive and/orerosive materials; for instance, pump casings and impellers, stuffingbox liners, the tips of mixers in soda ash apparatus, Shafts where theyrun through bushings in milk of lime reservoirs, shovel teeth used atquarries, spiral conveyor walls and flights, etc.

I claim: l l. The process for producing a hard surface on a ferrousmetal, which comprises applyinga layer offinely divided silicon carbideto the surface, subjecting the silicon carbide to an electric arc, ofwhich the ferrous metal forms one electrode and a carbon rod forms theother electrode, so as to fuse a layer of the metal in contact with thesilicon carbide and to dissolve silicon carbide in the fused metal.

2. The process for producing a hard surface on carbon'steel, whichcomprises applying a layer about fi to about A; inch thick of finelydivided silicon carbide to the surface, subjecting the silicon carbideto an electric arc, of which the steel forms one electrode and a carbonrod forms the other'electrode, so as to fuse a layer of the steel,between 0.05. and 0.4 inch in thickness but less than the thickness of.the steel, in contact with the silicon carbide and to dissolve siliconcar bide in the fused steel 1f 3. The process for producinga hardsurface on a carbon steel containing not more than about 0.4%of's'i1iconand not more than about 0.4% of carbon, which comprises applying a layerof about 1% inch to about inch thick of finely divided silicon carbideto the surface, subjecting thesilicon' carbide to an electric arc, ofwhich the'steel forms one electrodeand a carbon rod forms the otherelectrode, so as to fuse'a layer of the steel, between 0.05 inch and 0.4inch in thickness, but less than the thickness of the steel,

